Chiller suction flow limiting with input power or motor current control

ABSTRACT

A chiller includes an evaporator, a compressor including a prime mover, a first pressure sensor that detects a first pressure in the evaporator, a second pressure sensor that detects a second pressure in a condenser, and a controller. The controller determines a predicted energy level of the compressor based on the first pressure and the second pressure, the predicted energy level associated with liquid droplet flow into the compressor, compares the predicted energy level to an operating energy level, and modifies the at least one of the input power and the input current to the prime mover based on the comparison satisfying a modification condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Application of PCT InternationalApplication No. PCT/CN2018/089063, entitled “CHILLER SUCTION FLOWLIMITING WITH INPUT POWER OR MOTOR CURRENT CONTROL,” filed May 30, 2018,which is herein incorporated by reference in its entirety for allpurposes.

BACKGROUND

Refrigerants can transfer heat between fluids and may be employed in avariety of applications, such as heating, ventilating, and airconditioning (HVAC) systems, heat pumps, or power generation in OrganicRankine Cycles (ORC). The refrigerant can be transported within arefrigerant piping system, which includes pipes, pipe fittings, valves,and the like. The refrigerant piping system transports the refrigerantbetween various vessels and equipment within the HVAC system, such ascompressors, turbines, pumps, evaporators, and condensers. Therefrigerant may undergo one or more phase changes within the refrigerantpiping system, such that liquid refrigerant and vaporous refrigerant mayboth be present in the HVAC system.

SUMMARY

One implementation of the present disclosure is a chiller. The chillerincludes an evaporator that receives a first flow of refrigerant,transfers heat to the first flow of refrigerant, and outputs a secondflow of refrigerant. The chiller includes a compressor that receives thesecond flow of refrigerant via tubing between the evaporator and thecompressor, the compressor including a prime mover that performs work onthe second flow of refrigerant based on at least one of an input powerto the prime mover and an input current to the prime mover. The chillerincludes a first pressure sensor that detects a first pressure ofrefrigerant in the evaporator. The chiller includes a second pressuresensor that detects a second pressure of refrigerant in a condenser ofthe chiller. The chiller includes a controller that determines apredicted energy level of operation of the compressor based on the firstpressure and the second pressure, the predicted energy level associatedwith liquid droplet flow in the second flow of refrigerant received bythe compressor, compares the predicted energy level to an operatingenergy level of the compressor, and modifies the at least one of theinput power and the input current to the prime mover based on thecomparison satisfying a modification condition.

Another implementation of the present disclosure is a method of chillersuction flow limiting. The method includes receiving, by a controller, afirst pressure from an evaporator pressure sensor coupled to anevaporator. The method includes receiving, by the controller, a secondpressure from a condenser pressure sensor coupled to a condenser. Themethod includes determining, by the controller, a predicted energy levelof operation of a compressor based on the first pressure and the secondpressure, the predicted energy level associated with liquid droplet flowfrom the evaporator to the condenser. The method includes comparing, bythe controller, the predicted energy level to an operating energy levelof the compressor. The method includes modifying at least one of aninput power and an input current to a prime mover of the compressorbased on the comparison satisfying a modification condition.

Still another implementation of the present disclosure is a chillercontroller. The chiller controller includes one or more processors and amemory device storing computer-readable instructions that when executedby the one or more processors, cause the one or more processors toreceive, at a state detector, a first pressure from an evaporatorpressure sensor coupled to an evaporator; receive, at the statedetector, a second pressure from a condenser pressure sensor coupled toa condenser; determine, by an energy predictor, a predicted energy levelof operation of a compressor based on the first pressure and the secondpressure, the predicted energy level associated with liquid droplet flowfrom the evaporator to the condenser; compare, by a compressorcontroller, the predicted energy level to an operating energy level; andmodify, by the compressor controller, at least one of an input power andan input current to a prime mover of the compressor based on thecomparison satisfying a modification condition.

Those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, inventive features, and advantages of the devices and/orprocesses described herein, as defined solely by the claims, will becomeapparent in the detailed description set forth herein and taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a building serviced by a heating,ventilation, and air conditioning (HVAC) system, according to anexemplary embodiment.

FIG. 2 is a block diagram illustrating a portion of the HVAC system ofFIG. 1 in greater detail, showing a refrigeration circuit configured tocirculate a refrigerant between an evaporator and a condenser, accordingto an exemplary embodiment.

FIG. 3 is a block diagram of a controller of the refrigeration circuitof FIG. 2, according to an exemplary embodiment.

FIG. 4 is a flow diagram of a method of chiller suction flow limitingwith input power or motor current control, according to an exemplaryembodiment.

DETAILED DESCRIPTION

The present disclosure relates generally to the field of refrigerationsystems. More particularly, the present disclosure relates to chillersuction flow limiting with input power or motor current control. Arefrigeration system can include a chiller, which can include anevaporator, condenser, compressor, and tubing connecting these andvarious other components. The evaporator evaporates refrigerant toprovide net cooling of process fluid, such as water, flowing through thetubing. It can be desirable for the evaporator to generate a dry,saturated vapor from the refrigerant, and for the compressor to thusreceive the dry, saturated vapor based on suction generated by thecompressor. However, in some situations, the refrigerant outputted bythe evaporator includes liquid droplets, which are pulled up with thehigh velocity vapor flow based on the suction from the compressor. Inaddition, size, weight, power, and cost considerations may make itdesirable to reduce the size of the evaporator to meet the minimum needsof a design capacity of the refrigeration system. However, as lift ordifferential pressure across the compressor is lowered due to chilleroperating conditions, the compressor may provide a higher capacity andsuction flow rate, which can increase gas velocity in the evaporator andcause liquid droplets to carry over into the compressor. These effectscan decrease the efficiency of the chiller, and can damage mechanicalcomponents of the compressor.

The present solution can address such considerations by implementingchiller suction flow limiting with input power or motor current control,in order to effectively manage compressor operation to reduce oreliminate liquid droplet flow from the evaporator to the compressor. Forexample, systems and methods in accordance with the present solution canpredict power or current levels corresponding to a design velocity limitof the evaporator, at which liquid droplet flow to the compressor couldbe expected to occur, and use a controller to limit further power orcurrent increase to prevent the liquid droplet flow to the compressor(e.g., liquid flow carryover). In some embodiments, a chiller includesan evaporator that receives a first flow of refrigerant, transfers heatto the first flow of refrigerant, and outputs a second flow ofrefrigerant. The chiller includes a compressor that receives the secondflow of refrigerant via tubing between the evaporator and thecompressor, the compressor including a prime mover that performs work onthe second flow of refrigerant based on at least one of an input powerto the prime mover and an input current to the prime mover. The chillerincludes a first pressure sensor that detects a first pressure ofrefrigerant in the evaporator. The chiller includes a second pressuresensor that detects a second pressure of refrigerant in a condenser ofthe chiller. The chiller includes a controller that determines apredicted energy level of operation of the compressor based on the firstpressure and the second pressure, compares the predicted energy level toan operating energy level associated with liquid droplet flow in thesecond flow of refrigerant received by the compressor, and modifies theat least one of the input power and the input current to the prime moverbased on the comparison satisfying a modification condition. As such, ifthe predicted energy level is too high (e.g., is greater than theoperating energy level), the controller can limit the power or current,as appropriate, to the prime mover to reduce or eliminate a risk ofliquid droplet flow into the compressor, which might otherwise occur ifa design velocity limit of the evaporator is exceeded.

HVAC System

FIG. 1 depicts a perspective view of a building 10. Building 10 isserviced by a heating, ventilation, and air conditioning system (HVAC)system 20. HVAC system 20 can include a chiller 22, a boiler 24, arooftop cooling unit 26, and a plurality of air-handling units (AHUs)36. HVAC system 20 uses a fluid circulation system to provide heatingand/or cooling for building 10. The circulated fluid may be cooled inchiller 22 or heated in boiler 24, depending on whether cooling orheating is required. Boiler 24 may add heat to the circulated fluid byburning a combustible material (e.g., natural gas). Chiller 22 may placethe circulated fluid in a heat exchange relationship with another fluid(e.g., a refrigerant) in a heat exchanger (e.g., an evaporator). Therefrigerant removes heat from the circulated fluid during an evaporationprocess, thereby cooling the circulated fluid.

The circulated fluid from chiller 22 or boiler 24 may be transported toAHUs 36 via piping 32. AHUs 36 may place the circulated fluid in a heatexchange relationship with an airflow passing through AHUs 36. Forexample, the airflow may be passed over piping in fan coil units orother air conditioning terminal units through which the circulated fluidflows. AHUs 36 may transfer heat between the airflow and the circulatedfluid to provide heating or cooling for the airflow. The heated orcooled air may be delivered to building 10 via an air distributionsystem including air supply ducts 38 and may return to AHUs 36 via airreturn ducts 40. HVAC system 20 can include a separate AHU 36 on eachfloor of building 10. In other embodiments, a single AHU (e.g., arooftop AHU) may supply air for multiple floors or zones. The circulatedfluid from AHUs 36 may return chiller 22 or boiler 24 via piping 34.

The refrigerant in chiller 22 can be vaporized upon absorbing heat fromthe circulated fluid. The vapor refrigerant may be provided to acompressor within chiller 22 where the temperature and pressure of therefrigerant are increased (e.g., using a rotating impeller, a screwcompressor, a scroll compressor, a reciprocating compressor, acentrifugal compressor, etc.). The compressed refrigerant may bedischarged into a condenser within chiller 22. In some embodiments,water (or another fluid) flows through tubes in the condenser of chiller22 to absorb heat from the refrigerant vapor, thereby causing therefrigerant to condense. The water flowing through tubes in thecondenser may be pumped from chiller 22 to a cooling unit 26 via piping28. Cooling unit 26 may use fan driven cooling or fan driven evaporationto remove heat from the water. The cooled water from cooling unit 26 maybe delivered back to chiller 22 via piping 30 and the cycle repeats.

FIG. 2 depicts a block diagram illustrating a portion of HVAC system 20,according to an exemplary embodiment. Chiller 22 can include arefrigeration circuit 42 and a controller 100. Refrigeration circuit 42can include an evaporator 46, a compressor 48, a condenser 50, and anexpansion valve 52. Compressor 48 may be configured to circulate arefrigerant through refrigeration circuit 42. Compressor 48 can beoperated by controller 100. Compressor 48 may compress the refrigerantto a high pressure, high temperature state and discharge the compressedrefrigerant into a compressor discharge line 54 connecting the outlet ofcompressor 48 to the inlet of condenser 50. The compressor 48 can be orinclude a screw compressor, a semi-hermetic screw compressor, orcompressor 48 is a hermitic or open screw compressor, for example.Compressor 48 can also be or include a scroll compressor, areciprocating compressor, a centrifugal compressor, or still anothertype of compressor.

Condenser 50 may receive the compressed refrigerant from compressordischarge line 54. Condenser 50 may also receive a separate heatexchange fluid from cooling circuit 56 (e.g., water, a water-glycolmixture, another refrigerant, etc.). Condenser 50 may be configured totransfer heat from the compressed refrigerant to the heat exchangefluid, thereby causing the compressed refrigerant to condense from agaseous refrigerant to a liquid or mixed fluid state. The coolingcircuit 56 can include a heat recovery circuit configured to use theheat absorbed from the refrigerant for heating applications. The coolingcircuit 56 can include a pump 58 for circulating the heat exchange fluidbetween condenser 50 and cooling unit 26. Cooling unit 26 may includecooling coils 60 configured to facilitate heat transfer between the heatexchange fluid and another fluid (e.g., air) flowing through coolingunit 26. The cooling unit 26 can include a cooling tower. The heatexchange fluid can reject heat in cooling unit 26 and return tocondenser 50 via piping 30.

The refrigeration circuit 42 can include a line 62 connecting an outletof condenser 50 to an inlet of expansion device 52. Expansion device 52can expand the refrigerant in refrigeration circuit 42 to a lowtemperature and low pressure state. Expansion device 52 may be a fixedposition device or variable position device (e.g., a valve). Expansiondevice 52 may be actuated manually or automatically (e.g., by controller100 via a valve actuator) to adjust the expansion of the refrigerantpassing therethrough. Expansion device 52 may output the expandedrefrigerant into line 64 connecting an outlet of expansion device 52 toan inlet of evaporator 46.

Evaporator 46 may receive the expanded refrigerant from line 64.Evaporator 46 may also receive a separate chilled fluid from chilledfluid circuit 66 (e.g., water, a water-glycol mixture, anotherrefrigerant, etc.). Evaporator 46 may be configured to transfer heatfrom the chilled fluid to the expanded refrigerant in refrigerationcircuit 42, thereby cooling the chilled fluid and causing therefrigerant to evaporate. The chilled fluid circuit 66 can include apump 68 for circulating the chilled fluid between evaporator 46 and AHU36. AHU 36 may include cooling coils 70 configured to facilitate heattransfer between the chilled fluid and another fluid (e.g., air) flowingthrough AHU 36. The chilled fluid may absorb heat in AHU 36 and returnto evaporator 46 via piping 34. Evaporator 46 may output the heatedrefrigerant to compressor suction line 72 connecting the outlet ofevaporator 46 with the inlet of compressor 48.

The chilled fluid circuit 66 can include a chilled fluid temperaturesensor 74 positioned along piping 32. Chilled fluid temperature sensor74 may be configured to detect a temperature T_(cf) of the chilled fluid(e.g., the leaving chilled liquid temperature, etc.) flowing withinpiping 32 between evaporator 46 and AHU 36. The refrigeration circuit 42can include a suction temperature sensor 76 positioned along compressorsuction line 72. Suction temperature sensor 76 may be configured todetect a temperature T_(suc) of the refrigerant flowing withincompressor suction line 72 between evaporator 46 and compressor 48(i.e., the temperature of the refrigerant entering compressor 48). Therefrigeration circuit 42 can include a suction pressure sensor 78positioned along compressor suction line 72. Suction pressure sensor 78may be configured to detect a pressure P_(suc) of the refrigerantflowing within compressor suction line 72 between evaporator 46 andcompressor 48 (i.e., the pressure of the refrigerant entering compressor48). The refrigeration circuit 42 can include a discharge temperaturesensor 80 positioned along compressor discharge line 54. Dischargetemperature sensor 80 may be configured to detect a temperature T_(dis)of the refrigerant flowing within compressor discharge line 54 betweencompressor 48 and condenser 50 (i.e., the temperature of the refrigerantexiting compressor 48). The refrigeration circuit 42 can include adischarge pressure sensor 82 positioned along compressor discharge line54. Discharge pressure sensor 82 may be configured to detect a pressureP_(dis) of the refrigerant flowing within compressor discharge line 54between compressor 48 and condenser 50 (i.e., the pressure of therefrigerant exiting compressor 48).

Refrigeration circuit 42 can include an evaporator pressure sensor 86that detects a pressure P_(evap) of the refrigerant flowing withinevaporator 46, and a condenser pressure sensor 88 that detects apressure P_(cond) of the refrigerant flowing within condenser 50.Sensors 86, 88 may be similar to sensors 78, 82; sensors 78, 82 mayrespectively be used to perform the functions of sensors 86, 88 relatingto measuring pressures associated with evaporator 46 and compressor 48as described further herein. Sensors 86, 88 may be positioned at variouspoints in or adjacent to evaporator 46 and condenser 50, respectively,to detect respective pressure P_(evap) and P_(cond).

Compressor 48 includes a prime mover 84 (e.g., a motor). The prime mover84 can be a fixed speed drive or a variable speed drive. Controller 100can control operation of prime mover 84, such as to transmit controlsignals to prime mover 84 to control a rotation speed, flow rate, orother operational parameter of compressor 48. The controller 100 cancontrol operation of prime mover 84 based on at least one of a power ora current corresponding to operation of compressor 48. Depending onoperational conditions in refrigeration circuit 42, liquid droplets ofrefrigerant may flow from evaporator 46 to compressor 48. Controller 100can control operation of prime mover 84 to reduce or eliminate liquiddroplet flow from evaporator 46 to compressor 48.

Chiller Suction Flow Limiting with Input Power or Motor Current Control

FIG. 3 depicts a block diagram of a refrigeration system 150 includingcontroller 100, according to an exemplary embodiment. Controller 100 cancontrol operation of compressor 48, such as to control operation ofcompressor 48 based on at least one of input power or motor currentcontrol. Power and current, such as input power, input current, motorpower, motor current, can include power or current to the refrigerationsystem 150 (e.g., to the chiller), power or current to a motorcontroller of the refrigeration system 150, power or current to a driveof compressor 48 (e.g., variable speed drive), power or current to amotor of compressor 48, or other power or current used to causecompressor 48 to move. Controller 100 can use power and current limitsto protect compressor 48 (e.g., prime mover 84), or limit buildingenergy usage, and such limits can be very stable in terms of controlmethodology (e.g., controller 100 need not rely on detected liquiddroplets as an input to a feedback control loop, and thus can preventliquid droplet carryover before it occurs). For example, controller 100can control a power of operation of a variable speed drive prime mover84, or can control a current of operation of a fixed speed drive primemover 84. Controller 100 can determine a predicted energy level ofoperation of compressor 48 at which suction flow from evaporator 46 tocompressor 48 might be expected to cause liquid droplets to flow intocompressor 48, compare the predicted energy level to an actual operatingenergy level of compressor 48, and determine to limit capacity ofcompressor 48 based on the comparison to protect compressor 48 fromliquid droplet flow.

The controller 100 can include a communications interface 102 and aprocessing circuit 104. Communications interface 102 may include wiredor wireless interfaces (e.g., jacks, antennas, transmitters, receivers,transceivers, wire terminals, etc.) for conducting data communicationswith various systems, devices, or networks. For example, communicationsinterface 102 may include an Ethernet card and/or port for sending andreceiving data via an Ethernet-based communications network. In someembodiments, communications interface 102 includes a wirelesstransceiver (e.g., a WiFi transceiver, a Bluetooth transceiver, a NFCtransceiver, ZigBee, etc.) for communicating via a wirelesscommunications network. Communications interface 102 may be configuredto communicate via local area networks (e.g., a building LAN, etc.)and/or wide area networks (e.g., the Internet, a cellular network, aradio communication network, etc.) and may use a variety ofcommunications protocols (e.g., BACnet, TCP/IP, point-to-point, etc.).

The communications interface 102 can facilitate receiving inputs fromvarious sensors. The sensors may include, for example, chilled fluidtemperature sensor 74 configured to detect the temperature of thechilled fluid at an outlet of evaporator 46, suction pressure sensor 78configured to detect the pressure of the refrigerant in compressorsuction line 72, discharge pressure sensor 82 configured to detect thepressure of the refrigerant in compressor discharge line 54, and/orother sensors of chiller 22 and/or HVAC system 20 (e.g., suctiontemperature sensor 76, discharge temperature sensor 80, chilled fluidtemperature sensor 74, etc.). Communications interface 102 may receivethe inputs directly from the sensors, via a local network, and/or via aremote communications network. Communications interface 102 may enablecommunications between controller 100 and compressor 48.

The processing circuit 104 can include a processor 106 and memory 108.Processor 106 may be a general purpose or specific purpose processor, anapplication specific integrated circuit (ASIC), one or more fieldprogrammable gate arrays (FPGAs), a group of processing components, orother suitable processing components. Processor 106 may be configured toexecute computer code or instructions stored in memory 108 (e.g., fuzzylogic, etc.) or received from other computer readable media (e.g.,CDROM, network storage, a remote server, etc.) to perform one or more ofthe processes described herein.

Memory 108 may include one or more data storage devices (e.g., memoryunits, memory devices, computer-readable storage media, etc.) configuredto store data, computer code, executable instructions, or other forms ofcomputer-readable information. Memory 108 may include random accessmemory (RAM), read-only memory (ROM), hard drive storage, temporarystorage, non-volatile memory, flash memory, optical memory, or any othersuitable memory for storing software objects and/or computerinstructions. Memory 108 may include database components, object codecomponents, script components, or any other type of informationstructure for supporting the various activities and informationstructures described in the present disclosure. Memory 108 may becommunicably connected to processor 106 via processing circuit 104 andmay include computer code for executing (e.g., by processor 106) one ormore of the processes described herein.

The memory 108 can includes various modules for completing processesdescribed herein. More particularly, memory 108 includes a statedetector 110, an energy predictor 112, and a compressor controller 114.While various modules with particular functionality are shown in FIG. 3,controller 100 and memory 108 may include any number of modules forcompleting the functions described herein. For example, the activitiesof multiple modules may be combined as a single module and additionalmodules with additional functionality may be included. The controller100 can further control other processes beyond the scope of the presentdisclosure, including but not limited to controlling operation ofvarious components of refrigeration system 150 based on a desired orexpected load condition.

State detector 110 can receive state data from various sensors ofrefrigeration system 150. For example, state detector 110 can receivepressure data from evaporator pressure sensor 86 and from condenserpressure sensor 88. State detector 110 can also receive temperature datafrom temperature sensors.

Energy predictor 112 can receive the state data from state detector 110and determine a predicted energy level of operation of compressor 48based on the received state data. The predicted energy level cancorrespond to at least one of compressor speed, compressor capacity,water flow rate, water temperature, suction volume flow rate, compressorperformance, motor performance, and starter performance.

Energy predictor 112 can determine the predicted energy level based on afirst pressure received by state detector 110 from evaporator pressuresensor 86 and a second pressure received by state detector 110 fromcondenser pressure sensor 88. Energy predictor 112 can execute an energyprediction function to calculate the predicted energy level.

The energy prediction function may include one or more calculationparameters that the energy predictor 112 can apply to the first pressureand second pressure to calculate the predicted energy level. The one ormore calculation parameters can be determined based on experimentaland/or simulation testing of operation of refrigeration system 150. Forexample, the one or more calculation parameters can be determined byidentifying energy levels associated with various values of evaporatorpressure and condenser pressure, and fitting a curve, function, or otherrepresentation to the energy levels based on the values of evaporatorpressure and condenser pressure. The energy levels may be identified byoperating the refrigeration system 150 (or a driveline thereof) atvarious operating conditions. The energy levels may be identified byoperating the refrigeration system 150 at part-load conditions, whichmay provide a more accurate representation of the behavior of therefrigeration system when the one or more calculation parameters areused to predict the energy levels. It will be appreciated thatcalculation parameter(s) determined for a first refrigeration system 150may be applied to various other refrigeration systems 150. The one ormore calculation parameters may be determined for a specificrefrigeration system using inputs such as capacity, water flow rates,and water temperatures, with feedback values such as evaporatorpressure, condenser pressure, suction volume flow rate, and inputcurrent (or input power), and executing an iterative process due todependencies between evaporator pressure (or saturation temperature),suction volume flow limit, capacity, and desired volume flow rate. Wherethe calculation parameters are determined based on a driveline of therefrigeration system 150 (e.g., to extrapolate the determinedcalculation parameters to other units having a similar driveline),performance parameters (e.g., compressor performance, motor performance,starter performance) can be determined based on boundary conditionvariables of compressor 48 (e.g., suction pressure, volume flow rate (ornon-dimensional flow rate, theta), and discharge pressure (ornon-dimensional head, omega)), to determine the corresponding inputcurrent (or input power). As such, if the values of the boundarycondition variables (e.g., volume flow rate or theta are) selected to beat the appropriate limit values, then the driveline calculations candirectly provide the data needed to determine the calculationparameters. It will be appreciated that the calculation parameters canbe determined using processing circuit 104, or a processing circuit of adevice remote from refrigeration system 150 (or from a driveline) thatis operated to identify relationships between evaporator pressure,condenser pressure, and liquid droplet flow.

Energy predictor 112 can select a particular energy prediction functionto execute based on an operating characteristic of compressor 48, whichmay be stored by energy predictor 112. The operating characteristic mayindicate whether prime mover 84 of compressor 48 operates in a variablespeed mode of operation or a fixed speed mode of operation. If theoperating characteristic indicates that prime mover 84 operates in avariable speed mode of operation, energy predictor 112 can select theenergy prediction function according to Equation 1:

$\begin{matrix}{E_{VSD} = {\left( {a + {b \cdot p_{evap}}} \right) \cdot \left( \frac{p_{cond}}{p_{evap}} \right)^{c}}} & {{Eqn}.\mspace{11mu} 1}\end{matrix}$

If the operating characteristic indicates that prime mover 84 operatesin a fixed speed mode of operation, energy predictor 112 can select theenergy prediction function according to Equation 2:

$\begin{matrix}{E_{FSD} = {\left( {a + {b \cdot p_{evap}}} \right) \cdot {\ln\left( {c + \frac{p_{cond}}{p_{evap}}} \right)}}} & {{Eqn}.\mspace{11mu} 2}\end{matrix}$

As such, energy predictor 112 can execute the appropriate energyprediction function using the calculation parameters and the first andsecond pressures (e.g., p_(evap)=first pressure, p_(cond)=secondpressure) to calculate the predicted energy level. It will beappreciated that the values of the calculation parameters can bedetermined by fitting curves of the form shown in Equation 1 or Equation2, as appropriate, to identified values of energy level as a function ofevaporator pressure and condenser pressure. An iterative optimizationprocess may be used to determine the values of the calculationparameters. The functions shown in Equation 1 and Equation 2 may belinearized (e.g., by taking a logarithm, such as the natural logarithm,of both sides of the respective equations) to reduce the computationalrequirements for determining the calculation parameters by enabling theuse of a linear fit method, such as linear least squares methods.

Compressor controller 114 can control operation of compressor 48 (e.g.,control operation of prime mover 84). Compressor controller 114 canoutput a control signal corresponding to a desired input power or inputcurrent to compressor 48, including to limit the input power or inputcurrent as appropriate. Compressor controller 114 can use the operatingcharacteristic of compressor 48 to determine whether to generate thecontrol signal to control the input power (e.g., if compressor 48operates in a variable speed mode of operation) or the input current(e.g., if compressor 48 operates in a fixed speed mode of operation).The compressor controller 114 may initially calculate the input power orthe input current based on input variables such as desired water flowrates, water temperatures, or other variables representative ofperformance of the refrigeration system 150. The compressor controller114 can limit the initially calculated input power or input current toreduce or eliminate liquid droplet flow into compressor 48.

Compressor controller 114 compares the predicted energy level determinedby energy predictor 112 to an operating energy level. The predictedenergy level may correspond to an energy level, given certain values ofevaporator pressure and condenser pressure, at which liquid droplet flowinto compressor 48 from evaporator 46 may be expected to occur. Forexample, the predicted energy level may correspond to an energy level atwhich a design velocity limit of evaporator 46 is exceeded, or at whichliquid droplet flow has been determined to occur through experimentaland/or simulation testing. The operating energy level can be a currentenergy level of compressor 48; as such, compressor controller 114 canuse the comparison to determine whether compressor 48 is operating at acondition which may exceed the predicted energy level at which liquiddroplet flow into compressor 48 from evaporator 46 may be expected tooccur.

Compressor controller 114 can measure at least one of actual inputcurrent and actual input power. For example, compressor controller 114can include an input current sensor, such as a current transformer, tomeasure actual input current. Compressor controller 114 can include aninput power sensor, such as a voltage sensor that can be used todetermined actual input power (e.g., based on the actual input currentand the actual input power). Compressor controller 114 can determine theoperating energy level based on the at least one of the actual inputcurrent and the actual input power.

Compressor controller 114 modifies at least one of the input power orthe input current to compressor 48 based on the comparison satisfying amodification condition. For example, if the predicted energy level is avalue that should not be exceeded, compressor controller 114 can limitthe at least one of the input power or the input current responsive tothe operating energy level exceeding the predicted energy level (e.g.,if operating energy level is greater than predicted energy level, limitthe at least one of the input power and the input current). If thepredicted energy level is set to a value that triggers limiting,compressor controller 114 can limit the at least one of the input powerand the input current responsive to the operating energy level equalingthe predicted energy level (e.g., if predicted energy level is equal tooperating energy level, limit the at least one of the input power andthe input current). Compressor controller 114 can calculate thepredicted energy level as at least one of a predicted input current anda predicted input power, such that compressor controller 114 can performthe comparison by comparing at least one of actual input current topredicted input current and actual input power to predicted input power.

If the comparison does not satisfy the modification condition, such asif the operating energy level is less than the predicted energy level,then compressor controller 114 can determine to not limit the inputpower or the input current; for example, compressor controller 114 cancontinue to monitor the first and second pressures; compressorcontroller 114 can determine to increase the at least one of the inputpower and the input current (if desired performance, such as water flowrates or water temperatures, is indicative of instructions to increasethe at least one of the input power and the input current). As such,where measured values of input power and/or input current are above thepredicted values, compressor controller 114 can reduce the operatingcapacity of compressor 48; where measured values of input power and/orinput current are below the predicted values, the operating capacity ofcompressor 48 (and thus refrigeration system 150) is not limited bysuction flow, and compressor controller 114 can control the at least oneof the input power and the input current by executing various processes,such as by controlling the at least one of the input power and the inputcurrent based on T_(cf) of the chilled fluid leaving evaporator 46 asdetected by chilled fluid temperature sensor 74 (e.g., by comparingT_(cf) to a desired value of T_(cf)). Compressor controller 114 canexecute capacity control of compressor 48 based on one or more ofvariation of compressor speed using a variable speed drive; compressorsuction flow dampers or pre-rotation vane flow throttling; compressordischarge variable geometry diffuser flow throttling; or a capacitycontrol slide valve (e.g., if compressor 48 includes a screwcompressor).

Compressor controller 114 can limit the at least one of the input poweror the input current by setting the at least one of the input power orthe input current to a previous value. For example, compressorcontroller 114 can maintain a database of power and current values.Responsive to determining to limit the input power or the input current,compressor controller 114 can retrieve a previous value of input poweror input current from the history, such as a previous value at a pointin time at which compressor controller 114 determined not to modify theat least one of the input power or the input current based on acorresponding previous predicted energy level.

Compressor controller 114 can maintain a database including evaporatorpressure, condenser pressure, input power, input current, predictedenergy level, and various other operational parameters, along with anindication that the comparison indicated that the input power or inputcurrent was to be limited.

Compressor controller 114 can output an alert indicating liquid dropletflow may be occurring based on the comparison. For example, compressorcontroller 114 can cause communications interface 102 to transmit thealert. The alert may include information such as the operationalparameters maintained in the database by compressor controller 114. Thealert may include an indication of a value of a performance variablecorresponding to the modification condition being satisfied, such as awater flow rate or water temperature resulting in a predicted energylevel associated with liquid droplet flow.

FIG. 4 depicts a method 400 of operating a refrigeration system (e.g., achiller), according to an exemplary embodiment. The method 400 can beperformed using the HVAC system of FIG. 1 and/or the refrigerationsystem 150 of FIGS. 2-3.

At 405, a first pressure is received by a controller from an evaporatorpressure sensor.

The first pressure can be representative of a pressure of refrigerantflowing through an evaporator. The evaporator can receive a first flowof refrigerant, transfer heat to the first flow of refrigerant, andoutput a second flow of refrigerant.

At 410, a second pressure is received by the controller from a condenserpressure sensor. The second pressure can be representative of a pressureof refrigerant flowing through the condenser.

At 415, the controller determines a predicted energy level of operationof a compressor. The compressor can receive the second flow ofrefrigerant via tubing between the evaporator and the compressor. Thepredicted energy level can be determined using an energy predictionfunction that uses the first pressure and the second pressure as inputsand evaluates the inputs using predetermined parameters. The compressorcan include a prime mover that performs work on the second flow ofrefrigerant based on at least one of an input power to the prime moverand an input current to the prime mover. The prime mover can include avariable speed drive that the controller drives using input power. Theprime mover can include a fixed speed drive that the controller drivesusing input current.

At 420, the controller compares the predicted energy level of operationof the compressor to an operating energy level. The predicted energylevel can be associated with liquid droplet flow in the second flow ofrefrigerant received by the compressor. The predicted energy level cancorrespond to a design velocity limit of the evaporator. The compressorcontroller can determine the operating energy level using at least oneof an actual input current and an actual input power.

At 425, the controller determines whether the comparison satisfies amodification condition. The modification condition can correspond to theoperating energy level being greater than the predicted energy level, orthe operating energy level being greater than or equal to the predictedenergy level.

At 430, the controller limits at least one of the input power and theinput current to the prime mover based on the comparison satisfying amodification condition. The controller can modify the at least one ofthe input power and the input current to limit the at least one of theinput power and the input current to a value at which liquid dropletflow from the evaporator into the compressor can be reduced oreliminated. The controller can output an alert responsive to modifyingthe at least one of the input power and the input current to the primemover based on the comparison satisfying the modification condition. Ifthe comparison does not satisfy the modification condition, such as ifthe operating energy level is less than the predicted energy level, thenthe controller can continue to monitor the pressures received from theevaporator pressure sensor and the condenser pressure sensor. If thecomparison does not satisfy the modification condition, such as if theoperating energy level is less than the predicted energy level, thecontroller can continue to increase the input power or input current, asappropriate, if the desired performance of the chiller (e.g., desiredwater flow rates or water temperatures) indicate instructions toincrease the input power or input current.

References to “or” may be construed as inclusive so that any termsdescribed using “or” may indicate any of a single, more than one, andall of the described terms. References to at least one of a conjunctivelist of terms may be construed as an inclusive OR to indicate any of asingle, more than one, and all of the described terms. For example, areference to “at least one of ‘A’ and ‘B’” can include only ‘A’, only‘B’, as well as both ‘A’ and ‘B’. Such references used in conjunctionwith “comprising” or other open terminology can include additionalitems.

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although onlyexample embodiments have been described in detail in this disclosure,many modifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.). For example, the position of elements can bereversed or otherwise varied and the nature or number of discreteelements or positions can be altered or varied. Accordingly, suchmodifications are intended to be included within the scope of thepresent disclosure. The order or sequence of any process or method stepscan be varied or re-sequenced according to alternative embodiments.Other substitutions, modifications, changes, and omissions can be madein the design, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.

What is claimed is:
 1. A chiller, comprising: an evaporator configuredto receive a first flow of refrigerant, transfer heat to the first flowof refrigerant, and output a second flow of refrigerant; a compressorconfigured to receive the second flow of refrigerant via tubing betweenthe evaporator and the compressor, the compressor including a primemover configured to perform work on the second flow of refrigerant basedon at least one of an input power to the prime mover and an inputcurrent to the prime mover; a first pressure sensor configured to detecta first pressure of refrigerant in the evaporator; a second pressuresensor configured to detect a second pressure of refrigerant in acondenser of the chiller; and a controller configured to: determine apredicted energy level of operation of the compressor based on the firstpressure and the second pressure, wherein operation of the compressor atthe predicted energy level is expected to cause liquid droplet flow inthe second flow of refrigerant received by the compressor; determine anactual operating energy level of the compressor; and modify the inputpower or the input current to the prime mover based on comparison of thepredicted energy level and the actual operating energy level.
 2. Thechiller of claim 1, wherein the controller is configured to determinethe predicted energy level using an energy prediction function that usesthe first pressure and the second pressure as inputs and evaluates theinputs using predetermined parameters.
 3. The chiller of claim 1,wherein the prime mover comprises a variable speed drive, wherein thecontroller is configured to operate the variable speed drive.
 4. Thechiller of claim 1, wherein the prime mover comprises a fixed speeddrive, wherein the controller is configured to operate the fixed speeddrive.
 5. The chiller of claim 1, wherein a first maximum operatingcapacity of the compressor is greater than a second maximum operatingcapacity of the evaporator.
 6. The chiller of claim 1, wherein thecontroller is configured to output an alert responsive to modifying theinput power or the input current.
 7. The chiller of claim 1, wherein thecontroller is configured to modify the input power or the input currentbased on a determination that the actual operating energy level meets orexceeds the predicted energy level.
 8. A method of chiller suction flowlimiting, comprising: receiving, via a controller, a first pressure froman evaporator pressure sensor coupled to an evaporator; receiving, viathe controller, a second pressure from a condenser pressure sensorcoupled to a condenser; determining, via the controller, a predictedenergy level of operation of a compressor based on the first pressureand the second pressure, wherein operation of the compressor at thepredicted energy level is expected to cause liquid droplet flow from theevaporator to the compressor; determining, via the controller, an actualoperating energy level of the compressor; and modifying an input poweror an input current to a prime mover of the compressor based oncomparison of the predicted energy level and the actual operating energylevel.
 9. The method of claim 8, comprising determining, via thecontroller, the predicted energy level using an energy predictionfunction that uses the first pressure and the second pressure as inputsand evaluates the inputs using predetermined parameters.
 10. The methodof claim 8, comprising driving, via the controller, a variable speeddrive of the prime mover using the input power.
 11. The method of claim8, comprising driving, via the controller, a fixed speed drive of theprime mover using the input current.
 12. The method of claim 8, whereina first maximum operating capacity of the compressor is greater than asecond maximum operating capacity of the evaporator.
 13. The method ofclaim 8, comprising outputting, via the controller, an alert responsiveto modifying the at least one of the input power or the input current.14. A chiller controller, comprising: one or more processors; and amemory device storing computer-readable instructions that when executedby the one or more processors, cause the one or more processors to:receive, at a state detector, a first pressure from an evaporatorpressure sensor coupled to an evaporator; receive, at the statedetector, a second pressure from a condenser pressure sensor coupled toa condenser; determine, via an energy predictor, a predicted energylevel of operation of a compressor based on the first pressure and thesecond pressure, wherein operation of the compressor at the predictedenergy level is expected to cause liquid droplet flow from theevaporator to the compressor; determine, via a compressor controller, anactual operating energy level of the compressor; and modify, via thecompressor controller, at least one of a power or a current to a primemover of the compressor based on comparison of the predicted energylevel and the actual operating energy level satisfying a modificationcondition.
 15. The chiller controller of claim 14, wherein the energypredictor is configured to determine the predicted energy level using anenergy prediction function that uses the first pressure and the secondpressure as inputs and evaluates the inputs using predeterminedparameters.
 16. The chiller controller of claim 14, wherein the chillercontroller is configured to operate a variable speed drive of the primemover using the power or operate a fixed speed drive of the prime moverusing the current.
 17. The chiller controller of claim 14, wherein thecompressor controller is configured to output an alert responsive tomodifying the power or the current.